The nitrogen-vacancy (NV) center in diamond has a number of exceptional properties, including a long room-temperature spin coherence time (˜1 ms), optical accessibility for spin initialization and detection, and embedding in a solid-state spin environment that can be engineered for a wide variety of applications. One application for which the NV center has shown particular promise is quantum sensing including magnetometry. A standard method of sensing magnetic fields with NV centers uses an optically detected magnetic resonance (ODMR) spectrum to determine the NV ground state transition frequencies which experience Zeeman splitting as a function of the applied magnetic field and are scaled only by fundamental constants. However, current NV magnetometers that employ this method rely on measurements of NV fluorescence intensity and are therefore susceptible to noise in the optical and microwave excitation sources used to perform ODMR measurements.
An earlier approach to ODMR-based NV magnetometry measured the full spectra and performed fits to extract the NV resonance frequencies. It involved spending a large fraction of the measurement time monitoring non-information-containing off resonance signal and was subsequently prohibitively slow.
Another earlier method used lock-in techniques to continuously monitor a single resonance on the approximately linear derivative of the curve, from which small resonance frequency shifts were detected by applying pre-calibrated scale factors. However, this second approach was still limited to the approximately linear regime of the lock-in signal resulting in a dynamic range of about 10 μT. Furthermore, this method was dependent on phenomenological variables instead of a true frequency shift. In particular, the scale factor was influenced by the NV resonance linewidth and contrast, both of which were affected by optical pump power, microwave power, and detection efficiency. These variables are different for each device and drift over time, consequently requiring periodic calibration.
A third earlier method employed a closed-loop feedback lock-in technique on a single resonance of a single NV center; however such an approach is not inherently suited to magnetic sensing, suffering from severe systematic errors due to, for example, time-varying temperature and magnetic field when applied to magnetic sensing. Furthermore, single-NV measurements are incompatible with vector magnetometry. In particular, none of the previous methods performed measurements on multiple NV resonances in an NV ensemble simultaneously; thus none were able to extract full magnetic field vector information in real-time, decoupled from the effects of temperature.